For Review Only · alkylbenzenesulphonic acid (LABSA) was neutralized using analytical grade sodium hydroxide to obtain the anionic surfactant sodium linear alkylbenzenesulphonate

For Review OnlyBiodegradability of Single and Mixed Surfactant

Formulations

Journal: Songklanakarin Journal of Science and Technology

Manuscript ID SJST-2018-0427.R1

Manuscript Type: Original Article

Date Submitted by the Author: 21-Mar-2019

Complete List of Authors: Tayag, Jarrent; Angeles University Foundation, Graduate School; Ateneo de Manila University, School of Science and Engineering; Angeles University Foundation, College of EducationFabicon, Ronaldo; Ateneo de Manila University, School of Science and Engineering

Keyword: Environmental and Natural Resources, Chemistry and Pharmaceutical Sciences, surfactant biodegradability, surfactant combination

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Songklanakarin Journal of Science and Technology SJST-2018-0427.R1 Tayag

For Review Only

Original Article

Biodegradability of Single and Mixed Surfactant Formulations

Jarrent R. Tayag1,2,3 and Ronaldo M. Fabicon1

1School of Science and Engineering, Ateneo de Manila University, Quezon City,

Philippines

2Graduate School, Angeles University Foundation, Angeles City, 2009, Philippines

3College of Education, Angeles University Foundation, Angeles City, 2009, Philippines

*Corresponding author, Email address: [email protected]

Abstract

While detergent surfactants are mandated to be biodegradable, the environmental

fate of these surfactants when mixed together in bodies of water is still not established.

The study aimed to determine the biodegradability of NaLAS/CTAB surfactant

combinations by measuring the amount of evolved CO2. The amount of CO2 evolved

from the system was measured using the OECD 301b procedure. The 90/10 and 10/90

NaLAS/CTAB system showed a decline on their biodegradation behavior recording

55.88% and 40.12% biodegradation after the 28-day monitoring period. Conductivity

results revealed changes in the availability of ions in the system. An inflection point was

observed at 700ppm CTAB concentration. The highest turbidity is noted at 1.39:1

NaLAS/CTAB molar ratio indicating the formation of insoluble catanionic salts in the

system. Conductivity and turbidity testing revealed the formation of anionic-cationic

structures such as micelles and ion-ion complexes. These structures may alter the natural

degradation mechanism of microorganisms, thus, leading to the slower rate of

biodegradation or incomplete degradation of the surfactants.

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Keywords: surfactant biodegradation, surfactant combinations, waste water treatment

1. Introduction

The biodegradation of organic substances ensures that the environment does not

accumulate with organic contaminants. Defined as the breakdown of organic compounds

through the action of microorganisms, it is an important mechanism for the irreversible

removal of aquatic and terrestrial contaminants (Mishra & Tyagi, 2006).

Surfactants have high environmental importance because of their large production

volume. Of the 2260 kt surfactants demand in 2005, 55.5% ended up in household

applications such as soaps and detergents (Qin, Zhang, Kang, & Zhao, 2005). The

presence of surfactants in water system has led to the noted deterioration of the

organoleptic property of water. Furthermore, undesirable effects of surfactants in

wastewater include the reduction of the concentration of dissolved oxygen because of

foam formation and concomitant destruction of flora and fauna on surface water brought

about by enhanced eutrophication (Qin et al., 2005).

The formation of foams and suds in water systems and the consequent negative

effects to the organisms prompted the preference for biodegradable surfactants over their

non-biodegradable counterparts. Microbes cannot breakdown branched

alkylbenzenesulfonates, leading to their replacement to the straight-chained linear

alkylbenzenesulfonates (Qin et al., 2005). The formulation of fabric softening

composition with faster biodegradation resulted in the production of quaternary

ammonium salts containing ester groups. These “esterquats” have been tested to have

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rapid and complete biodegradation alongside their improved softening properties (Mishra

& Tyagi, 2006; Qin et al., 2005).

Over the years, different surfactants have undergone international recognized

screening and simulation tests to determine their biodegradation behavior (Garcia,

Campos, Marsal, & Ribosa, 2009). However, existing literature on the biodegradability

of surfactants is limited to reports of biodegradability behavior of a single surfactant

system. Tests are conducted to determine whether a particular surfactant would be

biodegradable given a certain concentration in the environment.

In reality, commercial compounding is done to obtain better effects among

surfactants. Surfactants are mixed with other surfactants to achieve synergistic gains.

Because of the more pronounced interfacial properties of mixed surfactant systems, these

have been used prevalently to improve detergency, emulsification and medicine

(Raghavan, Fritz, & Kaler, 2002; Homendra & Devi, 2006). For example, complexes of

different types of anionic, nonionic and cationic surfactants have been used in fabric

softener and liquid detergent formulations. Aside from these, builders and copolymers

are likewise added to these mixed systems to further improve their detergency property.

However, complex structures formed from compounding may affect their

biodegradability behavior. These complex structures also find their way in sewage which

further complicates their biodegradation. The formation of stable structures like

admicelles and micelles can interfere in the capacity of microorganisms to biodegrade

these compounds.

In this light, the purpose of this study is to determine the biodegradation of

surfactants and mixed surfactant systems in surface water. The study used sodium linear

alkylbenzene sulfonate (NaLAS), a common anionic surfactant and cetyl

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trimethylammonium bromide (CTAB), a cationic surfactant. Aerobic biodegradation

screening test was conducted to measure the amount of evolved CO2 from the systems

after a 28-day monitoring period. The results were used to determine whether the

surfactant systems are readily biodegradable under the set conditions. The presence of

anionic-cationic structures in the systems was also determined using conductivity test.

The study aimed to determine the biodegradability of single surfactants and

anionic-cationic surfactant system by measuring the amount of CO2 evolved through a

period of 28 days. The results were compared to assess the possible effects of mixed

surfactants to the biodegradation of individual surfactants. The mixed surfactant system

was then analyzed using conductivity testing to determine a possible change in the

structures formed within the system.

2. Materials and Methods

2.1. Materials

LAS and CTAB were among the commonly used surfactants for detergents at

present. The LAS molecule contains an aromatic ring sulfonated at the para position and

attached to a linear alkyl chain at any position except the terminal carbons. The linear

alkyl carbon chain typically has 10 to 14 carbon atoms, with the approximate mole ratio

varying regionally with weighted averages of 11.7-11.8. The alkyl chains are

predominantly linear, ranging from 87% to 98%. Commercial LAS consists of more than

20 individual components (OECD SIDS, 2005). Hexadecyltrimethylammonium bromide

(CTAB) is a bacterial, cationic detergent, which can be neutralized by soaps and anionic

detergents such as sodium dodecyl sulfate (SDS). Trimethylammonium bromide

compounds form insoluble complexes with SDS (Sigma Aldrich). CTAB has a reported

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critical micelle concentration (cmc) of 0.92 to 1.0 mM in water. CTAB is biodegradable

in aerobic conditions with about 97% biodegradation in several hours under the shaking

flask method (Qin et al., 2005).

The anionic surfactant linear alkylbenzenesulphonic acid (LABSA), with 96%

active matter, was purchased from Chemline Scientific and manufactured by Sigma-

Alrich. The cationic surfactant cetyltrimethylammonium bromide (CTAB) with 99%

purity was likewise obtained from Chemline Scientific. The analytical grade reagents

potassium dihydrogen orthophosphate, dipotassium hydrogen orthophosphate, disodium

hydrogen orthophosphate dehydrate, ammonium chloride, calcium chloride, magnesium

sulphate heptahydrate and iron (III) chloride hexahydrate were obtained from the

stockroom of the Department of Chemistry, Ateneo de Manila University.

2.2. Aerobic Biodegradation Screening Test

The study employed Organisation for Economic Co-operation and Development

(OECD) 301b procedure to test the biodegradation of the sample in aerobic conditions.

This procedure measures the amount of carbon dioxide evolved from the surfactant

system. During the biodegradation of surfactants, their alkyl chains are degraded by

microorganisms to produce carbon dioxide through ω- and β-oxidation.

This test involved the aeration of a measured volume of inoculated mineral

medium in diffuse light. Degradation is determined through the measurement of produced

carbon dioxide over 28 days. Barium hydroxide is used to trap the evolved CO2 and is

measured by titration of the residual hydroxide. This is made possible by the reaction of

barium hydroxide to the evolved CO2 resulting to the formation of barium carbonate and

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water. Barium carbonate appears as white, insoluble powder in the flask. The reaction

proceeds through the following equation:

Ba(OH)2 + CO2 BaCO3 + H2O

The amount of CO2 produced from the test substance, after having been corrected

from the CO2 derived from the blank inoculum, is expressed as a percentage of theoretical

CO2 yield.

2.2.1 Preparation of Inoculum and Samples

The inoculums used for this study is surface water taken from Marikina River.

Sample collection was assisted by the Laguna Lake Development Authority (LLDA). The

river water was stored and aerated in the laboratory in diffuse light for 5 days.

Preconditioning of the inoculum in the experimental conditions improves the precision of

the test method by reducing blank values.

Commercial NaLAS was not available during the conduct of the study, thus, linear

alkylbenzenesulphonic acid (LABSA) was neutralized using analytical grade sodium

hydroxide to obtain the anionic surfactant sodium linear alkylbenzenesulphonate

(NaLAS). To prepare 45% NaLAS, 29.45 grams of NaOH was dissolved in 150 mL

distilled water. 180 grams of distilled water was added to 250 grams of 96% LABSA in

a beaker while submerged in an ice bath. The NaOH solution was gradually added into

the LABSA. The mixture was stirred continuously until it became a viscous brownish

paste.

Four surfactant systems were prepared: 100% NaLAS, 90%NaLAS-10%CTAB,

10%NaLAS-90% CTAB and 100% CTAB. All flasks had a total surfactant concentration

of 500ppm. 100% NaLAS was prepared by dissolving 1.111 grams of 45% NaLAS in 1

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liter distilled water. 90%NaLAS-10%CTAB was prepared by dissolving 1 gram 45%

NaLAS and 0.05 grams CTAB in 1 liter distilled water.10%NaLAS-90%CTAB was

prepared by dissolving 0.111 grams 45% NaLAS and 0.45 grams CTAB in 1 liter distilled

water. 100% CTAB was prepared by dissolving 0.5 grams CTAB in 1 liter distilled water.

The mineral medium stock solutions were prepared through the following: a) 8.50

grams KH2PO4, 21.75 grams K2HPO4, 33.4 grams Na2HPO4.2H2O and 0.5 grams NH4Cl

were dissolved in 1 liter distilled water; b) 27.5 grams anhydrous CaCl2 was dissolved in

1 liter distilled water; c) 22.5 grams MgSO4.7H2O was dissolved in 1 liter distilled water;

and d) 0.25 grams FeCl3.6H2O was dissolved in 1 liter distilled water. To prepare the

mineral medium, 45 mL of stock A was mixed with 4 liters distilled water. 4.5 mL of

stocks B, C and D was then added to the solution. Distilled water was added to make up

4.5 liters solution.

A stock solution of the titrant, HCl was standardized using 0.1M NaOH as a

primary standard. 0.1225 grams of potassium hydrogen phthalate was dissolved in 20 mL

distilled water. This solution was titrated to ascertain the concentration of the NaOH

solution. 0.1M NaOH was then used to titrate 20 mL HCl. The end point was reached

with 22.6 mL 0.1M NaOH reading a concentration of 0.113M HCl solution. The Ba(OH)2

solution, which will serve as the CO2 absorber, was also standardized using the 0.113M

HCl solution. In the end, 5 liters of 0.113M HCl and 0.0986M Ba(OH)2 stock solution

were prepared for the experiment.

2.2.2 Preparation of Flasks

Four flasks containing the surfactant combinations were prepared. For each flask,

480 mL mineral medium and the surfactant combinations were mixed. The flasks were

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inoculated to give a final volume of 1 liter. A fifth flask was prepared without the

surfactant system to serve as an inoculum control. The flasks were connected to the

aerators, set at 100 mL/min, using rubber tubes. The flasks were stored in the laboratory

with diffused light. For each flask, 3 absorption bottles were connected. Each absorption

bottle contained 20 mL 0.0986M Ba(OH)2 solution. Each container was fitted with a

serum bottle closure. Three runs were done simultaneously.

2.2.3 Evolved CO2 Measurement

OECD 301b recommended the analyses of CO2 be done every second or third day

during the first 10 days and at least every fifth day until the end of the 28-day period. This

is to identify the 10-day window where substances are expected to reach the pass level of

60% biodegradation to be considered readily biodegradable. Because of this, the CO2

measurements were conducted 12 times in the 28-day period: 1st, 2nd, 3rd, 5th, 7th, 9th, 11th,

13th, 17th, 21st, 25th and 28th day. During the measurement days, the Ba(OH)2 absorber

closest to the flask was disconnected and titrated using 0.113M HCl solution using

phenolphthalein as the indicator. The remaining absorbers were then moved one place

closer to the flask and placed a new absorber containing 20 mL 0.0986M Ba(OH)2 at the

far end of the series.

The weight of CO2 evolved in each absorber is calculated through the following:

𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐶𝑂2 =0.113

𝑚𝑜𝑙𝑒1000𝑚𝐿 × (35𝑚𝐿 ― 𝑚𝐿 𝐻𝐶𝑙 𝑡𝑖𝑡𝑟𝑎𝑡𝑒𝑑) × 44

𝑔𝑟𝑎𝑚𝑚𝑜𝑙𝑒

2

where: 0.113 refers to the molarity of HCl used; 35 refers to the amount in mL of

0.113M HCl needed to titrate 20 mL 0.0986M Ba(OH)2; 44 refers to the molecular

(1)

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weight of CO2; and 2 refers to the number of mmol of HCl needed for the titration of

the remaining Ba(OH)2

The CO2 recorded from the blank was deducted from the CO2 readings of the other

flasks. The % degradation was computed using the following formula:

% 𝑑𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 = 𝐶𝑂2 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑

𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝐶𝑂2 𝑦𝑖𝑒𝑙𝑑

The theoretical CO2 yield for 100% NaLAS, 90%NaLAS-10%CTAB, 10%NaLAS-

90%CTAB, and 100% CTAB were 0.758 grams, 0.7786 grams, 0.9448 grams, and 0.966

grams, respectively.

2.3 Characterization

2.3.1 Conductivity Testing

Conductance measurement was carried out on a digital conductivity meter

(Oakton Conductivity Wand) with a sensitivity of 0.1% and a dipping type conductivity

cell with a platinum electrode cell constant 1.0. All the measurements were made at

ambient temperature (250C). Conductivity testing of the surfactant systems was done at

dilute concentration from 100ppm – 1000ppm. Two sets of dilute solutions were

prepared. For the first set, the NaLAS concentration was made constant at 500ppm. This

was prepared by dissolving 1.11 grams 45% NaLAS in 1 liter distilled water. A stock

solution of 1000ppm CTAB was also prepared by dissolving 1 gram CTAB in 1 liter

distilled water. From this, dilutions from 100ppm – 900ppm, in 100ppm interval, were

prepared. An equal volume of each surfactant solution was added to make a volume of

100mL for each testing. After measuring the conductivity of the 10 mixtures, the CTAB

concentration was made constant at 500ppm. The NaLAS concentration was varied from

(2)

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100ppm-1000ppm. The conductivity readings were plotted to identify possible deflection

points indicating changes in the behavior of ions in the solutions.

2.3.2 UV Vis Spectroscopy

Absorbance measurements were taken using Shimadzu 1800 double beam

spectrophotometer using 10-mm path length quartz cuvettes, which is available at the

Angeles University Foundation. The diluted concentrations prepared for the UV Vis

testing were the same samples used for the conductivity testing. Each sample was tested

at 200-600 nm wavelength range. Turbidity test result was taken from the absorption of

the samples at 470 nm. The absorbance was plotted against the concentration of the

samples from 100ppm – 1000ppm.

3. Results and Discussion

3.1. Biodegradation Profile of Surfactant Systems

The aerobic biodegradability of surfactant systems are assessed using the OECD

301b (CO2 Evolution Test). A compound is considered readily biodegradable if the

biodegradation level exceeds 60% within 28 days. It is assumed that such compound will

rapidly and completely biodegrade under aerobic conditions. The biodegradation curve

shows the increasing amount of evolved CO2 from the flasks because of the degradation

of the test substance. Upon reaching 10% degradation, rapid biodegradation is expected

with the microorganisms having adapted to the test material. At this point, the 10-day

window can be identified, which is the period where 60% biodegradation must be

achieved for the substance to be considered readily biodegradable. The 10-day window

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must end before the 28-day monitoring period. The curve ends with a plateau indicating

a decrease in the evolved CO2 from the flask.

Low values from the screening test do not necessarily mean that the test substance

is not biodegradable under environmental conditions. It simply indicates that more work

is necessary to establish its biodegradability. Likewise, if the plot does not plateau after

28 days, the test substance may not be considered readily biodegradable. 100%

biodegradation is not expected in this method as some of the carbon become part of the

microorganisms’ biomass. Figure 1 shows the rate of biodegradation of the four surfactant

systems.

----------------------

Figure 1

----------------------

Figure 1 shows the % biodegradation of the surfactant systems throughout the 28-

day monitoring period. As expected, none of the systems achieved 100% biodegradation.

The amount of CO2 measured through titration was subtracted from the CO2 reading from

the control (flask without the test materials) and compared with the theoretical CO2 yield

for each of the surfactant systems. There is an increasing amount of evolved CO2 from

day 1 until 15 with the plateau appearing from day 17 to day 28. There is an apparent

similarity with the biodegradation curves of the four surfactant systems because of the

presence of similar alkyl chains from the surfactants used. It is from these alkyl chains

that CO2 evolves in the systems.

After 28 days, NaLAS has reached 81.79% biodegradation, while CTAB recorded

61.12% biodegradation. Passing 60% biodegradation, these surfactant systems are

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considered readily biodegradable in nature. This conforms to existing literature (Scott &

Jones, 2000) stating that both NaLAS and CTAB are biodegradable under aerobic

conditions. CTAB apparently biodegrades longer compared to NaLAS because it has

longer alkyl chains. Biodegradation starts from one end of the alkyl chain, thus, the longer

the chain is, the longer time it will take for all the carbon links to be broken.

The mixed surfactant systems reached 55.88% biodegradation for 90-10

NaLAS/CTAB and 40.12% for 10-90 NaLAS/CTAB. The latter system recorded a lower

biodegradation rate because of the greater concentration of CTAB in the system. While

these systems have not achieved the pass level for the screening test, these mixed

surfactant systems cannot be classified as non-biodegradable in nature. The results imply

that the systems are not readily biodegradable and require further simulation tests to fully

describe their biodegradation behavior.

The suppression of the biodegradation of the mixed surfactant systems may be

because of the interaction of the surfactants with each other. The interaction of surfactants

in mixed systems can alter the physic-chemical properties of the system as revealed in the

studies conducted by Raghavan et al. (2002) and Homendra and Devi (2006). The

interaction of the alkyl chains of the oppositely charged headgroups can likewise modify

the behavior of the system as reported by Kohler, Raghavan, and Kaler (2000).

3.2. Determination of Cationic – Anionic Structures

3.2.1. Conductivity of Mixed Surfactant Systems

Conductivity measurements are widely used to characterize ionic surfactants such

as NaLAS and CTAB. The change in electrical conductance of aqueous ionic surfactant

solutions at the critical micelle concentration (cmc) is because of the difference in the

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degree of surfactant ionization below and above the cmc. Below the cmc, surfactant

monomers behave as strong electrolytes. Likewise is observed above the cmc where the

micelles have been partly ionized (Dominguez, Fernandez, Gonzalez, Iglesias, &

Montenegro, 1997). Because of the change in the conductance of the solution, a deflection

is observed in the conductance plot indicating the concentration at which the micelles are

formed. Figure 2 shows the conductivity measurements of the NaLAS/CTAB system with

NaLAS concentration at 500ppm.

-----------------------

Figure 2

----------------------

The specific conductance values at constant concentration (500ppm) of NaLAS

were obtained by varying the concentration of CTAB from 100-1000ppm. The critical

micelle concentration (cmc) of NaLAS as reported from literature (Berna et al., 2007) is

at 100ppm. At 500ppm, concentration above the cmc, micelles and monomers of NaLAS

are present in the solution (Dominguez et al., 1997). NaLAS at 500ppm at 0 ppm of CTAB

recorded a conductivity of 735 μS. The addition of 100ppm CTAB into the solution

caused the conductivity to drop to 414 μS. The monomers of NaLAS present in the

solution, which formerly contribute to the conductance of the solution, have formed ion-

ion complexes with the cationic CTAB. This resulted to the decrease in the conductivity

of the solution. Further addition of CTAB into the solution increases the ion-ion

complexes in the system and decreases the free ions. However, the conductivity of the

solution increases because of the increasing concentration of the surfactants in the system.

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The continual addition of surfactants causes an increase in the free ions in the system, as

indicated by rising slope.

The conductivity of the solution linearly increases with the concentration of the

surfactants. An inflection point is seen at 700ppm. Beyond the inflection point, the slope

indicates an increase in the conductivity of the solution, but with an apparent decrease in

the increment. The addition of CTAB in the solution resulted in the formation of ion-ion

complexes, which reduced the conductivity of the system. The lowering of the

conductivity beyond the inflection point is indicative of the formation of micelles.

Micelles can drastically lower the conductivity of the solution because they are only

partially ionized (Diaz & Velasquez, 2007).

Figure 3 shows the conductivity of the solutions with constant CTAB (500ppm)

mixed with varying concentrations of NaLAS. CTAB has a cmc at 365ppm (Scott &

Jones, 2000). Thus, at 500ppm, micelles of CTAB are already present in the system.

Because the concentration is above the known cmc for CTAB, monomers are also found

in the system, which accounts for the conductivity reading of the solution. The addition

of NaLAS (100ppm) initiated the formation of ion-ion complexes, similar to that in the

Figure 2. However, the inflection point for this combination is noted at a lower

concentration as opposed from that when NaLAS was set constant at 500 ppm. The cmc

of mixed surfactant systems is observed between the cmc of the individual surfactants.

This may be attributed to the difference in the length of alkyl chains of the individual

surfactants. Because NaLAS has greater concentration (500ppm) and it has lower cmc

(100ppm), the inflection point appeared at a lower concentration (Tanhaei et al., 2013).

Similar to the results reflected in Figure 2, micelles are suggested to form beyond the

inflection point, indicating the lowering of the conductivity of the solution.

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---------------------

Figure 3

---------------------

3.2.2. Absorbance of Mixed Surfactant System

Turbidity testing is carried out to verify the presence of insoluble catanionic salts

formed by the ion-ion complexes in the system. This is done by testing the diluted samples

(100ppm – 1000ppm CTAB at constant 500ppm NaLAS) under UV-Vis Spectroscopy at

470nm. The absorbance of the samples at 470 nm were taken and plotted against

concentration as gleaned from Figure 4.

From 100ppm to 600ppm, the systems remained transparent as indicated by the

low absorbance of the systems. The gradually increasing absorbance from these

concentrations may be due to the increasing amount of suspended particulates, which may

account for the insoluble catanionic salts or ion-ion complexes. These are insoluble salt

formed by the interaction of the anionic and cationic headgroups (Raghavan et al, 2002).

The highest turbidity is noted at 700ppm (1.39:1 NaLAS/CTAB molar ratio). The

appearance of precipitates at equimolar ratio is consistent with the report of Raghavan et

al. (2002). The formation of catanionic salts is likely to happen because of the strong

interaction between the cationic and anionic headgroups and almost equal tail lengths of

the surfactants. The tail lengths of NaLAS (n = 12) and CTAB (n = 16) pack efficiently

in the crystalline lattice thereby inducing the formation of a stable precipitation in the

solution, hence, rendering the system turbid. Such case is not observed with surfactants

with asymmetric tail lengths, which shows a homogeneous phase throughout a range of

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compositions. The solution goes back to being transparent beyond 700ppm concentration

suggesting the presence of homogeneous micellar solutions over the composition range.

------------------

Figure 4

------------------

3.3. Biodegradability of Surfactant Mixed Systems

The biodegradation of substances in the environment ensures that organic

compounds will not accumulate in the ecosystems. With the large volume of surfactants

released into the environment from various sources, their biodegradation must be ensured.

However, the CO2 evolution test reveals that the biodegradation profile of surfactants

when mixed with other surfactants is suppressed. 90/10 and 10/90 NaLAS/CTAB are not

readily biodegradable as compared to their pure surfactant counterpart. While the

screening test is not conclusive that the tested surfactant systems are not biodegradable

in the environment, the change in the biodegradation profile may pose effects on the water

systems. With the absence of water treatment facilities in the country particularly in

residential areas, surfactants are directly released into the sewage systems or bodies of

water. As a result, surfactants face an inevitable interaction with other surfactants.

The change in the ionic conductance of the NaLAS-CTAB system was explained

through the formation of various structures such as micelles and ion-ion complexes. The

formation of these structures were also verified by the turbidity test showing the formation

of the homogeneous micellar solutions and insoluble catanionic salts at equimolar ratio.

The ion-ion complexes formed by the strong interaction of the headgroups of the cationic

and anionic surfactants was determined through IR spectroscopy.

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While the study takes the biodegradation of the tested systems at extreme

surfactant combinations (90-10 and 10-90 NaLAS/CTAB), an apparent suppression of

biodegradation level is already identified. This suppression has even led to the

determination that these mixed surfactant systems, relative to the 60% pass level of

OECD 301b, are not readily biodegradable in nature. From the characterization tests, at

these concentrations, micelles are formed in the solution and a small amount of ion-ion

complexes. The formation of these structures can be the reason for the suppressed

biodegradation of the mixed surfactant systems.

Scott and Jones (2000) explain that the structures that surfactants take and form

influence their properties as in the case of the branched alkylbenzenesulfonate and linear

alkylbenzenesulfonate. Branched alkylbenzenesulfonates biodegrade slowly in the

environment because microorganisms cannot breakdown the compound. The enzymes in

microorganisms that evolved to break hydrocarbons may be incapable of digesting these

structures. Instead of taking the ω- and β-oxidation mechanism, branched alkyl

surfactants degrade one carbon at a time through α-oxidation. This causes these

compounds to accumulate in the water system.

Similarly, the formation of micelles and ion-ion complexes in the mixed

surfactant systems must have altered the biodegradation behavior of the surfactants

leading to their suppressed biodegradation level.

4. Conclusions

The biodegradation of surfactant systems is suppressed as compared to the

individual surfactants. The NaLAS-CTAB system tested in this study did not pass the

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ready biodegradation level based on the screening test conducted. Conductivity testing

reveal the changes in the free ions available in the system which may be attributed to the

formation of catanionic salts and micelles at dilute concentrations. Turbidity test also

show the great increase in absorbance at equimolar ratio confirming the formation of

insoluble catanionic salts.

With this, the researcher concludes that the decline in the biodegradation of the

surfactant system may be due to the presence of different structures, which are formed by

the interaction of the surfactants at dilute concentrations. These structures may alter the

natural degradation mechanism of microorganisms, thus, leading to the slower rate of

biodegradation or incomplete degradation of the surfactants.

From this study, it is recommended that ultimate biodegradation tests can be

conducted to verify the biodegradability behavior of the mixed surfactant system under

aerobic conditions. Available simulation tests may be conducted to assess the ultimate

biodegradation of the mixed surfactant system. Furthermore, biodegradation studies can

be conducted for surfactant systems close to the equimolar ratio to verify the effects of

the ion-ion complexes to the biodegradation behavior of the surfactants.

Acknowledgments

The authors wish to acknowledge the Philippine Commission on Higher

Education (CHED) for the scholarship and research funding.

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0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100% NaLAS 90-10 NaLAS-CTAB 10-90 NaLAS-CTAB 100% CTAB

Measurement Schedule (Day)

Biod

egra

datio

n (%

)

Figure 1. Aerobic biodegradation curves at a total surfactant concentration of

500ppm

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0 100 200 300 400 500 600 700 800 900 1000400

450

500

550

600

650

700

750

CTAB Concentration (ppm)

Cond

uctiv

ity (µ

S)

Figure 2. Conductivity of NaLAS/CTAB system (constant NaLAS 500ppm)

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0 100 200 300 400 500 600 700 800 900 1000300

400

500

600

700

800

900

NaLAS Concentration (ppm)

Cond

uctiv

ity (µ

S)

Figure 3. Conductivity of NaLAS/CTAB system (constant CTAB 500ppm)

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0 100 200 300 400 500 600 700 800 900 10000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Concentration (ppm)

Abso

rban

ce

Figure 4. Plot of Absorbance versus Varying Concentration of CTAB at 500ppm constant NaLAS

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Documents

For Review Only · alkylbenzenesulphonic acid (LABSA) was neutralized using analytical grade sodium hydroxide to obtain the anionic surfactant sodium linear alkylbenzenesulphonate